BILLIONS of years ago, a tiny cyanobacterium cracked open a water molecule – and let loose a poison that wrought death and destruction on an epic scale. The microbe had just perfected photosynthesis, a process that freed the oxygen trapped inside water and killed early Earth’s anaerobic inhabitants.
Now, for the first time, geologists have found evidence of the crucial evolutionary stage just before cyanobacteria split water. The find offers a unique snapshot of the moment that made the modern world. With the advent of photosynthesis came an atmosphere dominated by oxygen and, ultimately, the diversity of life forms that we know today.
“This was the biggest change that ever occurred in the biosphere,” says Kevin Redding at Arizona State University in Tempe. “The extinction caused by oxygen was probably the largest ever seen, but at the same time animal life wouldn’t be possible without oxygen.”
Photosynthesis uses light and a source of electrons to generate energy and power an organism. In the world as we know it, that source of electrons is water, with oxygen the waste product. But there are no signs that oxygen was being formed when photosynthesis first appeared around 3.4 billion years ago, so early photosynthesisers probably scavenged electrons by splitting other molecules like hydrogen sulphide instead.
That had changed by about 2.4 billion years ago, when deposits of oxidised minerals tell us that oxygen was beginning to accumulate in the atmosphere. Photosynthesis as we know it had evolved.
To help work out how this happened, Woodward Fischer at the California Institute of Technology in Pasadena and his colleagues studied South African rocks that formed just before the 2.4-billion-year mark. Their analysis shows that although the rocks formed in the anoxic conditions that had prevailed since Earth’s formation, all of the manganese in the rock was deposited in an oxidised form.
In the absence of atmospheric oxygen, manganese needs some sort of catalyst to help it oxidise – it won’t react without a bit of help. The best explanation, say Fischer’s team, is that a photosynthetic organism was using manganese as an electron source. That left unstable manganese ions behind, which reacted with water to form the oxides. Fischer presented the findings at the American Geophysical Union’s conference in San Francisco on 6 December.
Every researcher contacted by New Scientist has hailed the significance of the study, in part because the evidence exactly matches what evolutionary theories have predicted.
A close look at today’s plants and algae shows that manganese oxidation is still a vital part of photosynthesis. Within their photosynthetic structures are manganese-rich crystals that provide the electrons to drive photosynthesis. The crystals then snaffle electrons from passing water molecules to restore their deficit. It is this electron raid that cracks open water molecules and generates the oxygen we breathe.
This complicated process must have had simpler roots. In 2007, John Allen at Queen Mary, University of London, and William Martin at the University of Düsseldorf, Germany, suggested one scenario (Nature, doi.org/bs65kb). They believe that modern photosynthesis was born when early cyanobacteria by chance floated into a watery environment rich in manganese, and quickly adapted to take advantage of the new source of electrons.
Later, because manganese is a relatively scarce resource that can’t be tapped indefinitely, the cyanobacteria evolved a different strategy. They incorporated manganese directly into their photosynthetic structures and used it as a rechargeable battery: draining it of its electrons, but allowing its supplies to be replenished by stealing electrons from another, more plentiful source – water.
What Fischer’s team has found is evidence of the initial step in this process: an anoxic environment rich in manganese that has been stripped of electrons and left in an oxidised state, almost certainly by primitive cyanobacteria. “There had to be some intermediate step in the evolutionary process,” says Redding.
“This is big news,” says Martin. He adds that we can expect publications in the near future that provide more evidence compatible with the theory. “But this somewhat more direct geochemical evidence is really exciting.”